Bandwidth is more important then you might think. Here is how to know if your stealing it, and how to prevent it from being stolen.
What Is Bandwidth?
Simply stated, bandwidth = data transfer. It's the traffic flowing in and out of your website. But, really, it's a bit more then that.
Let's say you have a web page that is 5 KB in size, and you are allowed 25 KB of bandwidth (that's a small amount, but it's for ease of math). If one person accesses that 5 KB file, then you now have 20 KB bandwidth left over. If a total of 5 people access that 5 KB file, then you have maxed out your bandwidth.
So what happens when you exceed your allotted bandwidth? It depends. On most free severs, it means that your site will shut down temporarily. The time that your site is shut down can vary, on tripod you will only be shut down for 1-3 hours, but on most sites you could be shut down for the remainder of the day, 24 hours, or the remainder of the month if you have exceeded your monthly bandwidth.
But what does this mean to someone paying for their web space? A lot more, usually exceeding your bandwidth doesn't mean shutting down your website, it means charging the owner more money for that month. And sometimes it can be a lot more.
Look at it like this, before you think about direct linking that image, when you steal bandwidth, you are taking a piece of their bandwidth every time someone access that file, and that means you are stealing money directly from the owner of that file. Yes, stealing money. Bandwidth cost money, even when you don't exceed your limit.
When someone direct links to files / images not located on their own web server, this is called bandwidth theft. For example, let's say you found this image located at http://www.somewebsite.com/someimage.gif, and you really wanted to use it on your website (something you should never do without written permission). Instead of downloading that image on your hard drive, and then uploading the image to your personal account, on your own server, you just embedded the image on your website by typing <img src="http://www.somewebsite.com/someimage.gif" />. That would be direct linking. That means that whenever someone accesses that image, on your website or theirs, the owner of somewebsite.com has to pay money for it.
What Happens When Someone Gets Caught?
Well, the main thing that could happen is the owner of the file will delete it and do one of two things; nothing, leaving the thief with a bunch of broken links, or replace the file with something they really don't want on their website. A fun thing to replace the image with is an image advertising your website.
The owner of the file can also report the thief to their web host, and they can, and usually will, shut down the website.
In extreme cases, a thief can be prosecuted. Bandwidth theft is illegal, and they can be charged with theft.
Preventing Bandwidth Theft
The most commonly direct link file is an image. Because of this, free servers like tripod already make it impossible to direct link images, replacing them with big "hosted by tripod.com" images if an attempt to direct link is made.
But with paid hosting, you need to find out if your server has the Apache Server's Mod Rewrite Module installed. If so, it can examine the URL requesting the file, and it will either allow the file to display for "allowed domains" or produce a broken link for unknown URL's.
This is not all that you need to do, but for a complete tutorial on this go to http://www.bignosebird.com/apache/a13.shtml. "Big Nose Bird" has some great tutorials, and more. This link will explain the prevention of direct linking, and also has more tutorials to check out.